Heretofore, most keyboards have been formed of an array of switches comprised of a plurality of discrete components that are united to form a relatively complex electro-mechanical linkage. Normally, such switches include a plunger which, when depressed, will complete or close an electrical circuit. Generally, a discrete spring, which is compressed when the plunger is depressed, is used to return the plunger to its normal position.
Keyboards of the type having discrete components united to form an electro-mechanical linkage have not proven to be satisfactory. One reason is because of the mounting of the individual plungers, springs and leverages in close relation becomes a tedious and time-consuming task. Also, these keyboards have a questionable level of reliability due to the large number of mechanical elements required for each switch and the wear associated with these mechanical elements. Further, the cost of such keyboards is relatively high.
To overcome the problems associated with keyboards using switches having discrete components, it has been proposed to provide the switches in large, relatively flat, plate-like banks. Such a keyboard is described in U.S. Pat. No. 3,600,528, wherein a plurality of movable contact elements are formed on a sheet of tempered beryllium copper. In one described form, the movable contact elements have a solid, flat center with a plurality of spider-like fingers extending radially outward for yieldable contact with a ring-like, fixed contact element formed on a printed circuit board. Each key is defined by spacers which extend between the fixed contact and the movable contact. By this arrangement, it is alleged that when a downward pressure is applied against the solid center, the spider legs associated therewith are flexed while the center contact moves spirally perpendicular to the fixed contact elements of the printed circuit board. Thus, it is alleged that a wiping contact is made between the movable contact element and the fixed contact element therebeneath, and that this contact is achieved with a minimum of cost in both assembly and material.
The relatively flat bank of switches provided by the structure of U.S. Pat. No. 3,600,528, is not satisfactory for a number of reasons. First, the formation of the plurality of spider-like fingers of each contact element requires substantial etching or gang stamping to remove a substantially large, interconnected portion of the metallic sheet. Specifically, four sets of four spider-like legs must be etched for each movable contact element. Obviously, the formation of this keyboard is complex and expensive. Also, since only a very small portion of the metallic sheet remains as the movable contact, the contact is weakly supported and thus prone to failure when subjected to repeated depressions. Partly to overcome this weakness, the movable contact elements are supported above the circuit board by a screen of spacer elements, thus adding to cost and complexity. Additionally, the movable contacts do not provide a good wiping contact with the stationary contacts since there is believed to be only a slight rotation of the movable contact when it is depressed. Also, the keyboard is not adaptable easily to multiple-switching actions.
Other electro-mechanical switches have been formed by cutting a pattern of apertures in an electrically conductive sheet. U.S. Pat. No. 3,697,711 teaches the use of arcs of a circle to provide a flexible switch, as does U.S. Pat. No. 3,467,923. However, these switches do not provide a sufficiently yieldable movable member such as is needed for the switches of a keyboard. In U.S. Pat. No. 3,594,522, a plurality of continuous spiral conductors are used in an elastic diaphragm switch, the spiral conductors being coupled to a central support. The continuous spiral path is very flexible, being supported at only one point, and hence they would not survive the repeated depressions required of a switch utilized in a keyboard.
In some switches of the type discussed, the closing of the electrical circuit is achieved by a mechanical contact between two contact elements. In another type of switch, a metallic plate is moved toward two separated, electrically energized, stationary plates to effect a change in the capacitive coupling between each of the stationary plates and the movable plate to thereby provide a capacitive switching action. Since the two separated, electrically energized plates act with the movable plate as capacitors in series, the change in capacitive coupling is very small. Attempts have been made to use capacitive switching in which a charged electrode of the capacitor is moved toward the other charged electrode of the capacitor. While this arrangement gives a greater capacitive coupling than one using capacitors in series, attachment of a lead to a moving member in a keyboard environment has not been feasible heretofore because, for example, of the close space requirements of conventional keyboards.